1. Field of the Invention
The present invention relates to a water-injection foaming devolatilizing method and apparatus for a polymer and, more particularly, to a novel improvement for efficiently removing volatile components from a polymer melt.
2. Related Art
Heretofore, as this type of water-injection foaming devolatilizing method and apparatus, there is generally used an extruder having such a configuation as shown in FIG. 6.
That is, in FIG. 6, the reference numeral 1 designates a casing of a twin screw type extruder 2, in which a water-injection port 3 and a vent port 4, on the downstream side of the water-injection hole 3, are provided on the upper surface of this casing 1, and a pair of screws 5 are rotatably provided in the cylinder 1 of the extruder 2 so as to be intermeshed with each other while overlapping each other.
In the aforementioned cylinder 1, a water-injection dispersing zone 10 having the aforementioned water-injection port 3, and a devolatilizing zoner 11 having the vent port 4 in the downstream side thereof are formed.
Part of the aforementioned screws 5 in the aforementioned water-injection dispersing zone 10 is constituted by a first ring 12, a plurality of kneading/dispersing screws 13, and a second ring 14. Part of the screws 5 in the devolatilizing zone 11 is constituted by a full flight screw 5a.
Next, in the aforementioned conventional configuration, under the condition that a polymer melt 20 is melted, kneaded and extruded by the screws 5, water 3a injected through the water-injection port 3 is dispersed, under a high pressure by the kneading/dispersing screws 13, into the high-temperature polymer melt 20 with which the water-injection dispersing zone 10 is filled. Further, this high pressure is maintained by the extruding function of the screw 5 in the upstream side of the first ring 12 and by the damming function of the second ring 14, so that the pressure is higher than the saturated vapor pressure of the water 3a which is dispersed into the high-temperature polymer melt 20. Accordingly, the water 3a is dispersed as liquid particles into the polymer melt 20. When the polymer melt 20 passes through the second ring 14, the pressure of the polymer melt 20 is reduced suddenly so that the water 3a dispersed into the polymer melt 20 is vaporized to thereby cause sudden foaming because the devolatilizing zone 11 at the downstream side of the second ring 14 is in a vacuum state. In this occasion, volatile components contained in the polymer melt 20 begin to be diffused into bubbles through a bubble/polymer interface from a point of time when a foaming phenomenon appears. Further, the bubbles in the foamed polymer melt 20 are broken down by the shearing function of the screws 5 in the devolatilizing zone 11 which is at the downstream side of the second ring 14. As a result, the volatile components contained in the bubbles are diffused to the outside of the polymer melt 20 and discharged to the outside from the vent port 4.
Because the conventional water-injection foaming devolatilizing method and apparatus is configured as described above, there are problems as follows. That is, as shown in FIG. 7, the pressure P of the polymer melt in the water-injection dispersing zone is reduced suddenly after the polymer melt passes through the second ring in the downstream side.
Accordingly, the generation, growth and breakdown of bubbles due to vaporization of water dispersed in the polymer melt mainly occur when the polymer melt passes through the second ring. Accordingly, the bubbles do not grow up sufficiently, so that the residence time thereof is short. As a result, not only the area of the bubble/polymer interface as a diffusive area concerning devolatilizing efficiency is relatively small but also the residence time of bubbles as a diffusive time is short, such that breakdown is not performed sufficiently.
Further, because the water dispersed into the polymer melt takes heat from the polymer melt around the water suddenly when the water is vaporized, the temperature of the interface film portion of the polymer melt around the generated bubbles becomes locally considerably lower than the temperature of the polymer melt, that is, the temperature of the interface film portion gets into a supercooling state. As a result, the diffusive velocity of volatile components which are diffused through this film layer is lowered.
Further, because the generation, growth and breakdown of bubbles occur concentratedly suddenly in a short time after the polymer melt passes through the secondring, there arises easily an entrainment phenomenon in which polymer splits are generated and absorptively discharged from the vent port.